1. Field of the Invention
The present invention is related to an LED lighting device, and more particularly, to an LED lighting device capable of providing synchronized PWM dimming control.
2. Description of the Prior Art
An LED lighting device directly driven by a rectified alternative-current (AC) voltage usually adopts a plurality of LEDs coupled in series in order to provide required luminance. LED lighting has been widely utilized in different application scenarios. To save energy or provide different brightness, dimming technologies have also been developed so that the lighting can be dimmed in different situations. Traditionally, there are different categories of dimming methods, including pulsed width modulation (PWM) dimming and analog dimming. Analog dimming changes LED light output by directly adjusting the DC current in the LED string, while PWM dimming achieves the same effect by varying the duty cycle of a constant current in the LED string to effectively change the average current in the LED string. A user may be provided with a means to control the LED dimming.
In the prior art, PWM dimming may be achieved by periodically switching on and off the LED current according to a PWM signal. The duty cycle of the LED current may thus be adjusted, thereby changing the overall luminance of an LED lighting device. However, if the frequency of the PWM signal is not synchronized with the frequency of the rectified AC voltage, the waveform of the LED current may vary during different cycles of the rectified AC voltage, thereby causing flicker or shimmer. LED flicker or shimmer, whether perceptible or not, has been a concern of the lighting community because of its potential human impacts, which range from distraction, mild annoyance to neurological problems. Therefore, there is a need for an LED lighting device capable of providing synchronized PWM dimming control.